1. Field of the Invention
The present invention relates to a rubber composition suitable for use as, for example, a tire, which is obtained by blending a gelled rubber synthesized with an acid anhydride etc., without substantially decreasing abrasion resistance, with the improved wet performance, and with the decreased rolling resistance, and a process for producing the same.
2. Description of the Related Art
In rubber compositions used for automobile tires etc., rubber compositions superior in drivability on wet road and low in rolling resistance are desired. From such a viewpoint, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-204217 proposes to blend an SBR rubber gel to a rubber composition. This SBR rubber gel is synthesized by a method of cross-linking the SBR rubber with a polyfunctional compound such as divinylbenzene during the polymerization or a method of cross-linking of the polymer after polymerization with a peroxide etc.
It is known that, when carbon is blended into rubber the tan xcex4 curve with respect to the temperature becomes broad and the viscoelastic properties are deteriorated. The present inventors engaged in research to develop a rubber composition having a superior wet performance, that is, having a high grip on wet road surfaces and having a low rolling resistance, by mixing a rubber having a low glass transition temperature Tg with a gelled rubber having a high glass transition temperature Tg so as to suppress the incorporation of carbon black to high Tg rubber and hopefully to obtain a rubber composition with the balanced values of tan xcex4 at 0xc2x0 C. and tan xcex4 at 60xc2x0 C. As a result, we found that, by mixing a gelled rubber obtained by gelation with a cross-linking agent in the diene rubber, the viscoelastic properties of the rubber composition can be improved. Further, we found that a rubber composition prepared by a two-stage mixing method of premixing the carbon black and other compounding agents to a low Tg rubber, and then mixing the resultant mixture with the gelled rubber obtained by gelation of a high Tg rubber further provides the improved viscoelastic properties.
Accordingly, the objects of the present invention is to provide a rubber composition having a superior tan xcex4 balance, without substantially decreasing the abrasion resistance and with the improved wet performance and decreased rolling resistance and a process for producing the same.
In accordance with the present invention, there is provided a rubber composition comprising 50 to 90 parts by weight of a diene rubber and 50 to 10 parts by weight of a gelled rubber having a toluene swelling index of 16 to 150, based upon the total amount of 100 parts by weight of the diene rubber and the gelled rubber.
In accordance with the present invention, there is also provided a process for producing a rubber composition comprising 100 parts by weight of a starting rubber containing 50 to 90 parts by weight of a diene rubber and 50 to 10 parts by weight of a gelled rubber, the glass transition temperature of the diene rubber being at least 10xc2x0 C. less than the transition temperature of the gelled rubber, and 30 to 120 parts by weight of a filler, comprising the steps of mixing the diene rubber and at least 80% by weight of the total amount of the filler at a temperature of at least 135xc2x0 C., and then mixing the gelled rubber and the remaining filler with the resultant mixture.
In a tire use rubber composition comprised of a low Tg (i.e. glass transition temperature) polymer and high Tg polymer, the tan xcex4 around 0xc2x0 C. is affected to a large extent by the compatibility of the two polymers. The tan xcex4 around 0xc2x0 C. is improved if the two polymers are incompatible. Further, by blending carbon black to the rubber, the tan xcex4 curve is broadened. Therefore, when the carbon black is predominated in the low Tg polymer and then the high Tg polymer is blended thereto, the incorporation of the carbon black into the high Tg polymer side is suppressed and the tan xcex4 around 60xc2x0 C. is decreased.
In the present invention, a gelled rubber having a toluene swelling index of 16 to 150 is blended, as the high Tg polymer, to a low Tg polymer of a diene rubber the incompatibility of the two polymers is promoted, the tan xcex4 around 0xc2x0 C. is improved, and the abrasion resistance is not too much impaired or, in some cases, is even improved. Further, according to the present invention, when the filler such as the carbon black is predominated in the low Tg polymer and when the gelled rubber is added thereto, the tan xcex4 around 0xc2x0 C. is improved and the tan xcex4 around 60xc2x0 C. is decreased and the abrasion resistance is maintained.
The gelled rubber used in the present invention may be produced by, for example, reacting a diene rubber such as a styrene-butadiene copolymer rubber (SBR) with maleic anhydride and a phenol compound having an aromatic hydroxyl group (e.g., phenol, t-butylphenol, catechol and t-butyl catechol, hydroquinone, and resorcinol etc.) or a peroxide compound (e.g., dicumyl peroxide etc.) or a mercapto compound (e.g., trithiocyanic acid, 1,10-decanethiol, etc.) at a temperature of 80 to 250xc2x0 C.
Examples of the above phenol compounds are as follows.
Monophenol Compounds
2,6-di-t-butyl-p-cresol butylated hydroxyanisole (BHA)
2,6-di-t-butyl-4-ethylphenol
Stearyl-xcex2-(3,5-di-t-butyl-4-hydroxyphenyl) propionate
Bisphenol Compounds
2,2xe2x80x2-methylenebis(4-methyl-6-t-butylphenol)
2,2xe2x80x2-methylenebis(4-ethyl-6-t-butylphenol)
4,4xe2x80x2-thiobis(3-methyl-6-t-butylphenol)
4,4xe2x80x2-butylidenebis(3-methyl-6-t-butylphenol)
3,9-bis[1,1-dimethyl-2-[xcex2-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraoxapyro[5,5]undecane
Phenol Polymer Compounds
1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane
1,3,5-trimethyl-2,4,6-tris (3,5-di-t-butyl-4-hydroxybenzyl)benzene
Tetrakis-[methylene-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4-hydroxyphenyl)propionate]methane
Bis[3,3xe2x80x2-bis(4xe2x80x2-hydroxy-3xe2x80x2-t-butylphenyl)butyric acid]glycol ester 
1,3,5-tris(3 xe2x80x2, 5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxybenzyl)-S-triazine-2,4,6-(1H,3H,5H)trione
Tocopherol(S)
Other Phenol Compounds
2,6-di-tert-butyl-4-methylphenol (BHT)
Mono(or di-or tri-)(xcex1-methylbenzyl)phenol
2,2xe2x80x2-methylenebis(4-methyl-6-tert-butylphenol)
2,2xe2x80x2-methylenebis(4-ethyl-6-tert-butylphenol)
4,4xe2x80x2-butylidenebis(6-tert-butyl-3-methylphenol)
4,4xe2x80x2-thiobis(6-tert-butyl-3-methylphenol)
1,1-bis(4-hydroxyphenyl)-cyclohexane
2,5-di-tert-butylhydroquinone
2,5-di-tert-amylhydroquinone
comprises the phenol compound abstraction of the hydrogen from the benzyl position or allyl position, adding the benzyl radicals or allyl radicals thus formed to the acid anhydride, and adding the radicals to the double bonds of other rubber molecules or radical coupling them with other rubber molecules to form cross-linking.
The gelled rubber according to the present invention has to have a toluene swelling index of 16 to 150, preferably 16 to 100. The xe2x80x9ctoluene swelling indexxe2x80x9d in the present invention is determined by immersing 0.1 g of a gelled rubber in 100 ml of toluene at room temperature for 24 hours, weighing the weight of the rubber at that time (wet weight), then drying the rubber in vacuo at room temperature for 24 hours, measuring the dry weight, and finding the swelling index from the wet weight/dry weight. If the toluene swelling index is too small, the abrasion resistance deteriorates, whereas if it is conversely too large, there is no great difference with normally ungelled rubber and no improvement effects in the viscoelastic properties are observed. Further, a gelled rubber containing at least 0.1% by weight, preferably 0.5 to 10% by weight, of an acid anhydride moiety in the molecule, has less deterioration of the abrasion resistance and in some cases is further improved compared with an ordinary rubber. Further, a rubber composition obtained by mixing the gelled rubber produced with a peroxide compound or mercapto compound to a diene rubber may be affected in vulcanized physical properties with the peroxide compound or mercapto compound, and therefore, a gelled rubber produced from an acid anhydride and phenol compound is more preferable.
The rubber composition according to the present invention is obtained by blending 50 to 90 parts by weight, preferably 60 to 85 parts by weight, of the diene rubber and 50 to 10 parts by weight, preferably 40 to 15 parts by weight, of at least one gelled rubber, (Note: the total amount of 100 parts by weight). If the amount
Examples of the other cross-linking agents are organic peroxides such as dicumyl peroxide, t-butylcumyl peroxide, bis-(t-butyl-peroxy-isopropyl)benzene, di-t-butyl peroxide, 2,5-dimethylhexane-2,5-dihydroperoxide, 2,5-dimethyl-3-hexine-2,5-dihydroperoxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate; organic azo compounds such as azobisisobutyronitrile and azobiscyclohexanenitrile; dimercapto compounds and polymercapto compounds such as dimercaptoethane, 1,6-dimercaptohexane, and 1,3,5-trimercaptotriazine, trimethylol propane tris(xcex2-thiopropionate), 1,8-dimercapto-3,6-dioxaoctane, mercapto-terminated polysulfide rubbers such as mercapto-terminated reaction products of bis-chloroethylformal and sodium polysulfide, sulfur chloride, dimercapto acids, quinones (e.g., p-quinone), quinonedioximes (e.g., p-quinonedioxime, p-quinoneoxime benzoate), polyhalides (trichloromelamine, hexachlorocyclopentadiene, octachlorocyclopentadiene, trichloromethanesulfochloride, benzotrichloride, paraffin chloride, PVC, chloroprene rubber, chlorosulfonated polyethylene, etc.), and metal oxides (stannous chloride, lead oxide, etc.), or triethanolamine, paraformaldehyde or polyoxymethylene and protonic acid or Lewis acid (stannic (II) chloride, paratoluene sulfonic acid, etc.), boranes (for example, triethylamine-chloroborane, triethylene diamine-bischloroborane); dinitrones (phenylhydroxylamine or terephthalaldehyde), dinitrile oxides (terephthalonitrile oxide, terephthalohydroxamyl chloride), dinitrile imines (terephthalylphenylhydrazide chloride and triethylamine), disydnones (p-phenylene-3,3xe2x80x2-disydnones), thionyl paraphenylene diamine, etc.
In the present invention, at least 0.1% by weight, preferably 0.5 to 30% by weight, of the maleic anhydride and at least 0.05% by weight, preferably 0.1 to 5% by weight, of a phenol compound such as tert-butyl catechol are reacted with the rubber. The reaction mechanism of the gelled rubber blended is too small, the expected effect is hard to appear, while if conversely too large, the abrasion resistance is deteriorated.
The diene rubber used in the rubber composition of the present invention is not particularly limited, but any diene rubber generally used in various types of rubber compositions in the past, such as natural rubber (NR), polyisoprene rubber (IR), styrene-butadiene copolymer rubber (SBR), polybutadiene rubber (BR), acrylonitrile butadiene copolymer rubber (NBR), butyl rubber (IIR), chloroprene rubber, ethylene-propylene copolymer rubber, ethylene-propylene diene copolymer rubber, etc. may be mentioned. These rubbers may be used alone or in any blend thereof.
The rubber composition according to the present invention is preferably produced as follows.
That is, according to the present invention, 30 to 120 parts by weight, preferably 40 to 100 parts by weight, of a filler such as carbon black or silica is blended to 100 parts by weight of a starting rubber comprised of 50 to 90 parts by weight, preferably 60 to 85 parts by weight, of a diene rubber and 50 to 10 parts by weight, preferably 40 to 15 parts by weight, of a gelled rubber. At that time, a diene rubber having a glass transition temperature Tg of at least 10xc2x0 C. lower than the Tg of the gelled rubber is preferable. More preferably, the diene rubber and at least 80% by weight of the total weight of the filler are mixed at a temperature of at least 135xc2x0 C., and then the gelled rubber and the remaining filler are mixed to produce the desired rubber composition.
The rubber composition may contain thereof compounding agents normally used in the rubber industry if necessary. As such compounding agents, in addition to a filler such as the above carbon black and silica, for example a vulcanization accelerator, vulcanization promoter, antioxidant, plasticizer, softener, etc. may be mentioned. These may be blended in their respectively necessary amounts.
The rubber composition according to the present invention may be used for various types of rubber products such as tires, hoses, conveyor belts, rubber sheets, and fenders, but is particularly preferably used as a rubber composition for a tire.